1. The Field of the Invention
The present invention relates to a method for locating magnetic structural bodies within the earth and in particular to a method using neural networks for determining the subsurface location, geometry and depth to basement of these bodies from aeromagnetic data.
The magnetic method plays an important role in mineral, petroleum, and geothermal exploration. It also has made important contributions to geologic mapping, structural geology, and platetectonic theory. In particular, magnetic measurements using aircraft (aeromagnetics) provide a relatively inexpensive way to trace magnetic rock units beneath covered areas, to reveal the shape of subsurface magnetic bodies, and to interpolate subsurface geologic information between widely spaced seismic data and other localized geophysical measurements.
Aeromagnetics are used by the petroleum industry to map magnetic features within the crystalline basement of the earth's crust such as geologic faults, magnetic bodies associated with mineral deposits, and basement structures associated with oil & gas accumulations. The aeromagnetics actually measure the magnetite content within the crystalline basement and from there interpretations are made about the geologic structure of the area. Aeromagnetics are relatively inexpensive data to collect and are used in the initial phases of petroleum exploration when no other or little data exists.
Interpretation of aeromagnetic data generally consists of modelling of the magnetic field data, either forward or inverse modelling, filtering and enhancement techniques of the mapped contour data, and profile analysis. The modelling procedure involves taking the individual aeromagnetic profile data and fitting a given geologic model (i.e. geologic cross-section) magnetic response to the actual collected profile data until the cross-section magnetic response matches the actual data. The filtering procedure involves computer filtering of the contour data (i.e. second vertical derivative, horizontal gradient, bandpass filtering, continuation filtering, inversion filtering, etc.) in order to interpret the underlying geologic bodies causing the measured magnetic response. This step is generally very subjective and does not always result in accurate interpretation of the underlying geology. Another interpretation technique is to manually analyze the individual magnetic flightline profiles for basement structure. This step is extremely labor intensive and most of the time is not done at all because it requires an experienced geophysicist to do. However, of all the procedures, profile interpretation is the most important. As one can see, these procedures are very laborious, convoluted, and time consuming and as a result, most of these interpretational procedures are never done or are done on a limited basis. Therefore if a procedure could be developed which could accurately identify basement structure and automate this laborious process, it would be very valuable to the petroleum industry. This is the basis of our current invention, to accurately identify basement structures and automate the time consuming processing neural networks.
Several excellent reviews of the current state of aeromagnetic interpretation techniques have been published by, for example: Reford, M. S., 1980, Magnetic Method: Geophysics, vol. 45, pp. 1640-58; Paterson, N. R., and Reeves, C. V., 1985, Applications of Gravity and Magnetic Surveys: The State-of-the-Art in 1985: Geophysics, vol. 50, pp. 2558-94; and Hinze, W. J., ed., 1985, The Utility of Regional Gravity and Magnetic Anomaly Maps, Tulsa, Okla., Society of Exploration Geophysicists, 454 pgs.
2. The Prior Art
Determination of the location and the depth to basement faults and structures in the earth is one of the most important reconnaissance exploration tools in the petroleum exploration industry. This is because many hydrocarbon reservoirs are associated with uplifted basement fault blocks and accurate mapping of subsurface structures greatly improves exploration well success rates and lowers the cost of finding hydrocarbons in sedimentary basins.
Subsurface geologic structure is commonly deduced from the mapping of surface structures and features, well log correlations, and by seismic reflection and refraction profiling. However, surface features do not always reflect deep structure when masked by surficial alluvium and moderately lithified shallow sediments. Also, the high cost of drilling deep exploratory well holes and collecting reflection seismic data often preclude their economic usefulness in delineating deep structures. Thus surveys which measure the magnetic field at or above the earth's surface, particularly from an airplane (aeromagnetics), can be an economic, environmentally attractive alternative to these other methods in unexplored or underexplored sedimentary basins.
Currently, aeromagnetic data are interpreted using combinations of simple computational and empirical techniques. The following Table I lists several of the manual and computer techniques currently used to predict location and depth of magnetic basement structural features.